1. Field of the Invention
The present invention relates to a sheet conveying apparatus, an image forming apparatus and an image reading apparatus and, more particularly, to a configuration of correcting skew feeding of sheets such as recording paper or document to be conveyed to an image forming portion or an image reading portion, and correcting misalignment of sheets in the width direction.
2. Description of the Related Art
Conventionally, image forming apparatuses and image reading apparatuses such as copying machines, printers and facsimiles have sheet conveying apparatuses which convey sheets such as recording paper or document to image forming portions or image reading portions. Further, some sheet conveying apparatuses have a skew feeding correcting portion which corrects skew feeding of sheets to adjust the posture and position of a sheet until it is conveyed to the image forming portion or image reading portion.
Recently, with, for example, an image forming apparatus, various sheets such as coated paper, embossed paper, ultra thick cardboard and ultra thin paper are used. Hence, the image forming apparatus is demanded to not only work more productively, but also increase the speed of correction of skew feeding and more accurately correct skew feeding to support all types of sheets to be used. In view of above, a skew feeding correcting portion is discussed adopting a system in which two pairs of skew feeding correction rollers are disposed at a predetermined interval in the width direction and which corrects skew feeding of a sheet by means of these pairs of skew feeding correction rollers while conveying the sheet without temporarily stopping the sheet in order to increase the speed of correction of skew feeding and more accurately correct skew feeding (Japanese Patent Laid-Open No. 4-277151).
Incidentally, these two pairs of skew feeding correction rollers of the skew feeding correcting portion causes unevenness in rotation (fluctuation of a conveying speed of sheets) due to phase resulting from respective eccentricities and outer peripheral shapes, and, in this case, a sheet is conveyed in a fluctuated manner due to the difference in unevenness in rotation between the skew feeding correction rollers. This fluctuation caused by the difference in unevenness in rotation between the skew feeding rollers cannot be controlled, and therefore there is an issue that, when a sheet reaches a conveying roller in the downstream while skew feeding of the sheet is corrected, the skew feeding amount corresponding to the difference in unevenness in rotation remains. In view of above, a configuration has been conventionally discussed which cancels the difference in unevenness in rotation and corrects skew feeding simultaneously by, for example, rotating conveying rollers with cutouts once and controlling the conveying roller (see U.S. Patent Application Publication No. 2008/006992 A1).
Further, recently, the demand for image formation on various sheets is increasing, and there are cases where images are formed on non-rectangular sheets such as tab sheets which are not necessarily rectangular. A “tab sheet” refers to a sheet having a tab at a side edge in which an index is written for the purpose of classification. However, the position of the tab is not fixed, and is provided by being shifted at several stages to facilitate confirmation of an index written in the tab. There are cases where the conventional skew feeding correcting portion cannot correct skew feeding of these sheets having non-rectangular shapes.
In view of above, as a skew feeding correcting method of correcting skew feeding of sheets having a non-rectangular shape like tab sheets, a method is discussed which corrects skew feeding of sheets by calculating a skew feeding amount based on sheet shape information and skew feeding detection sensor information registered in advance (Japanese Patent Laid-Open No. 2003-146485). More specifically, skew feeding of sheets is corrected by calculating the skew feeding amount based on, for example, information of two skew feeding detection sensors arranged in the width direction and sheet shape information (the dimension of a tab) registered in advance.
Further, another skew feeding correcting method includes detecting a shape of a front end of a sheet by means of a line sensor provided in the width direction to detect skew feeding of the sheet, when the sheet is determined to be a tab sheet, calculating the skew feeding amount of a tab sheet according to image processing and performing skew feeding correction suitable for the tab sheet.
However, with a sheet conveying apparatus which has this conventional skew feeding correcting portion, when, for example, skew feeding of a sheet is corrected according to sheet shape information, if there is a difference between sheet shape information input in advance and the shape dimension of a tab sheet which is actually conveyed, skew feeding occurs. Further, according to a method of detecting a sheet shape by means of a line sensor, particularly when there are rectangular sheets and tab sheets in a mixed manner, if the accuracy to detect skew feeding is improved or the speed of the entire apparatus is increased, image processing required to calculate the skew feeding amount becomes very enormous.
In light of the foregoing, the present invention provides a sheet conveying apparatus, an image forming apparatus and an image reading apparatus which can accurately correct skew feeding of non-rectangular sheets such as tab sheets.